1. Field of the Invention
The present invention relates to a current drive circuit where the output current, for example, is adjustable and the load is current-driven in pulse form in accordance with an external signal.
There is a light emitting element drive circuit for current driving a light emitting element such as, for example, a laser diode in pulse form as a current drive circuit. In addition, there is a digital-analog converter, as another example of a current drive circuit. This digital-analog converter has a configuration where a current value that corresponds to the weight of each bit of two-value data made of one or more bits is outputted from a current outputting part provided for each bit, an output current for each bit is turned on and off in accordance with the value of two-value data, and output currents for respective bits are added, synthesized and outputted.
2. Prior Art
In recent years, light emitting elements such as laser diodes have been current-driven in pulse form so that information is written into a memory medium such as an optical disk. It is necessary to increase the speed of the drive pulse of a light emitting element in order to increase the speed of such write-in of information.
A conventional light emitting element drive circuit for current-driving a light emitting element, such as a laser diode, is disclosed in, for example, Patent Document 1.
FIG. 11 is a circuit diagram showing a light emitting element drive circuit according to the prior art (Patent Document 1). In FIG. 11, symbol VREF indicates a reference voltage source. Symbols M14 and M15 indicate MOS transistors that form a current mirror-type current output circuit. Symbol M11 indicates a MOS transistor that forms a circuit for generating reference current IREF in accordance with reference voltage source VREF. Symbol PG indicates a pulse generation circuit. Symbol SW11 indicates a switch which turns on and off by receiving an output signal from pulse generation circuit PG. Symbol OS indicates a one-shot circuit which starts up by receiving an output signal from pulse generation circuit PG. Symbols M12 and M13 indicate circuits for improving a rise in the output current by receiving an output from one-shot circuit OS.
In this light emitting element drive circuit, switch SW11 is turned on when a current is outputted. Then, reference current IREF that has been generated by MOS transistor M11 and reference voltage source VREF is connected to the current mirror-type current output circuit that is formed of MOS transistors M14 and M15. Furthermore, the rise in the output current is improved by MOS transistors M12 and M13, as well as one-shot circuit OS.
When the current is turned off, switch SW11 is turned off. As a result of this, the gate potential of MOS transistor M15 is adjusted to a voltage which is lower than power supply voltage VDD by the threshold voltage of MOS transistor M14, by using MOS transistor M14.
In addition, a digital-analog converter is disclosed in, for example, Patent Document 2.
FIG. 12 is a circuit diagram showing a digital-analog converter according to the prior art (Patent Document 2). In FIG. 12, symbol IREF indicates a reference current source. Symbols M21 and M22 indicate MOS transistors that form a current mirror circuit for mirroring the current of reference source IREF. Symbols M24 and M25 indicate MOS transistors that form a current mirror-type current output circuit. Symbol SW21 indicates a switch for turning off MOS transistor M25.
In this digital-analog converter, switch SW21 is turned off at the time when a current is outputted. On the other hand, when the current is cut off, switch SW21 is turned on and MOS transistor M25 is turned off.
Patent Document 1: Japanese Unexamined Patent Publication 2003-188465 (page 11, FIG. 6)
Patent Document 2: Japanese Unexamined Patent Publication H11 (1999)-251912 (page 1, FIG. 1) Recording has conventionally been carried out on a recording medium such as an optical disk by using a light emitting element such as a laser diode. In recent years, high-speed recording in this recording medium which may be an optical disk has been required due to demand in the market. As a result of this requirement, it has become necessary to current-drive a light emitting element such as a laser diode at a high speed.
However, there has been a problem so far in the operation with high-speed rise and fall in the case where a light emitting element such as a laser diode is current-driven.
In addition, it has conventionally been necessary to optimize the amount of light emitted from the laser diode by means of adjustment of the amount of drive current to the light emitting element, in order to carry out optimal recording on a recording medium such as an optical disk. That is to say, the drive current to the light emitting element which may be a laser diode is not constant, but rather, is adjusted to the optimal value. Therefore, it has been necessary to drive a light emitting element drive circuit with the optimal amount of current in accordance with respective factors, such as the type of recording medium, the recording method and the properties of the light emitting element. Thus, there has been a problem so far in achieving high-speed rise and fall while maintaining the linearity of the output current.
In the case of a circuit such as that disclosed in FIG. 11, for example, first, switch SW11 is turned on when a current is outputted. Then, reference current IREF that has been generated by MOS transistor M11 and reference voltage source VREF is connected to the current mirror-type output circuit, which is formed of MOS transistors M14 and M15. In addition, the rise in the output current is improved by MOS transistors M12 and M13, as well as one-shot circuit OS.
When the output current is turned off, however, first, switch SW11 is turned off. Then, the gate potential of MOS transistor M15 is increased by using only MOS transistor M14 where a diode connection is made, and thereby, MOS transistor M15 is turned off. Accordingly, a high-speed fall cannot be expected.
In addition, the gate potential of MOS transistor M15 is provided by using MOS transistor M14 where a diode connection is made. Therefore, a voltage that is lower than power supply voltage VDD by the threshold voltage of MOS transistor M14 is provided. Accordingly, MOS transistor M15 is not completely turned off. As a result, a leak current flows.
In addition, in the case of a circuit such as that disclosed in FIG. 12, switch SW21 is turned on when the current is cut off. In this circuit, MOS transistor M25 is turned off by turning on switch SW21, and therefore, a high-speed fall can be achieved. However, MOS transistor M22 continues to be in the state of operation, and therefore, the gate potential of MOS transistor M25 does not become completely equal to power supply voltage VDD. Therefore, MOS transistor M25 cannot be completely turned off, and a leak current flows.
In addition, when a current is outputted, the current mirror circuit, which is a current output circuit and is formed of MOS transistors M24 and M25, is driven only by the current mirror circuit that is formed of MOS transistors M21 and M22. Here, a current drive circuit is driven by reference current source IREF. Accordingly, when the value of reference current IREF is small, relative to the input load capacitance of the current drive circuit, a high-speed rise in the output current cannot be expected.